The long term goal of Tang's research is to understand the molecular mechanisms underlining bacterial pathogenesis and host defense. Bacterial pathogens inject a set of proteins into the host cells through the type III secretion pathway that mediate bacterial pathogenesis and sometimes activate host defense. Most type III effector proteins produced by plant bacterial pathogens are as avirulence (Avr) proteins that induce the disease resistance in host plants carrying the corresponding disease resistance (R) genes. However, many avr genes enhance the bacterial fitness on host plants without the R genes. The two opposite functions of Avr proteins are fundamental to both bacterial pathogenesis and host defense. This proposal will use the avrPto gene of Pseudomonas syringae pv. tomato as a model system to study the molecular mechanisms that mediate the avirulence and virulence functions of Avr proteins. P. s. tomato(avrPto) causes the disease resistance in tomato plants carrying Pto, an R gene encoding a protein kinase. AvrPto physically interacts with Pto in the yeast two-hybrid system; this interaction is strictly correlated with the disease resistance in tomato plants carrying Pto. avrPto also enhances the pathogenicity of P. s. tomato on tomato plants lacking Pto. AvrPto mutants were identified that disrupt the avirulence but not the virulence, suggesting that the two opposite functions are structurally separated. To understand how AvrPto mediates virulence and host defense, the following objectives are proposed: 1) To characterize the AvrPto structure responsible for the virulence and avirulence functions through mutagenesis and NMR analyses; 2) To isolate and characterize tomato gene(s) targeted by AvrPto for virulence; 3) To identify tomato genes that are specifically regulated by the virulence and the avirulence functions of AvrPto. and 4) To characterize the AvrPto-Pto recognition in vivo. The proposed research will provide insight into the molecular mechanisms underlining bacterial pathogenesis and host defense.